A couple of questions about static classes and static members

I am reading a book and there is a listing in it with this code in it:

Code:

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import java.util.concurrent.*;
import java.util.concurrent.locks.*;

public class AccountWithSyncUsingLock {
  private static Account account = new Account();

  public static void main(String[] args) {
    ExecutorService executor = Executors.newCachedThreadPool();

    // Create and launch 100 threads
    for (int i = 0; i < 100; i++) {
      executor.execute(new AddAPennyTask());
    }

    executor.shutdown();

    // Wait until all tasks are finished
    while (!executor.isTerminated()) {
    }

    System.out.println("What is balance ? " + account.getBalance());
  }

  // A thread for adding a penny to the account
  public static class AddAPennyTask implements Runnable {
    public void run() {
      account.deposit(1);
    }
  }

  // An inner class for account
  public static class Account {
    private static Lock lock = new ReentrantLock(); // Create a lock
    private int balance = 0;

    public int getBalance() {
      return balance;
    }

    public void deposit(int amount) {
      lock.lock(); // Acquire the lock

      try {
        int newBalance = balance + amount;

        // This delay is deliberately added to magnify the
        // data-corruption problem and make it easy to see.
        Thread.sleep(5);

        balance = newBalance;
      }
      catch (InterruptedException ex) {
      }
      finally {
        lock.unlock(); // Release the lock
      }
    }
  }
}

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I can understand the proper use of static nested classes by reading this article : http://onjava.com/onjava/excerpt/Har...06/index2.html
What I cannot understand is why the author is using static keyword only for variables that refer to an object and not for primitive variables.
Thank you.

Quote:

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What I cannot understand is why the author is using static keyword only for variables that refer to an object and not for primitive variables.

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Which variable or variables do you think should be declared differently?

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I don't think the primitive vs reference type distinction is necessarily what is driving the author's choice.

I get it know.I've read this article here : » Blog Archive » Java Classes and Objects | Java Beginner
The phrase that solved all the above questions is this : 'Static variables can be accessed even though no objects of that class exist. '

What you should understand about static members in a class is that they are the same for every instance of that class.

They can be used on primitives too. Take this example as described in the Java Tutorials:

Code:

---

class Bicycle {

  public static int numOfBicycles = 0;
  public int bicycleId;

  //constructor
  Bicycle() {
    bicycleId = ++numOfBicycles;
  }

  //getters
  public int getId() { return bicycleId; }
  public static int getNumOfBicycles() { return numOfBicycles; }

}

---

Create a test class to test it out

Code:

---

...
public static void main(String[] args) {
    System.out.println(Bicycle.getNumOfBicycles()); //output = 0
    Bicycle myBike = new Bicycle();
    System.out.println(Bicycle.getNumOfBicycles()); //output = 1
    Bicycle hisBike = new Bicycle();
    System.out.println(Bicycle.getNumOfBicycles()); //output = 2

    System.out.printf("myBike id=%d%n", myBike.getId()); //output = myBike id=1
    System.out.printf("hisBike id=%d%n", hisBike.getId()); //output = hisBike id=2

}
...

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In each bicycle instance, the non-static bicycleId field is different, but the static field numOfBicycles is the same.